Prothrombin Time

International Normalized Ratio (INR), Pro Time, PT (Prothrombin Time)

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

• Taking medicines that can affect the action of blood thinners (such as warfarin) and vitamin K. These include antibiotics, aspirin, cimetidine (Tagamet), barbiturates, birth control pills, hormone replacement therapy (HRT), and vitamin K supplements.
• Having severe diarrhea or vomiting that causes fluid loss and dehydration. This may make the PT time longer. If diarrhea is caused by poor absorption of nutrients, vitamins, and minerals from the intestinal tract (malabsorption syndrome), the PT may be longer because of a lack of vitamin K.
• Eating foods that have vitamin K, such as beef liver, pork liver, green tea, broccoli, chickpeas, kale, turnip greens, and soybean products.
• Drinking a lot of alcohol.
• Taking laxatives.
• Taking some herbal products or natural remedies.

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Author:	Robin Parks, MS	Last Updated: September 15, 2008
Medical Review:	Anne C. Poinier, MD - Internal Medicine Brian Leber, MDCM, FRCPC - Hematology
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